1. Field of the Invention
The present invention relates to a plasma film forming method and apparatus and a plasma processing apparatus.
2. Description of the Related Art
With the development of higher-integration semiconductor devices, the circuit configurations of the devices have become more complicated with respect to three-dimensional directions. Accordingly, superfine circuits with half-micron- or submicron-order pattern widths are expected to have a multilayer structure. In this circumstance, multilayer wiring is a very important technique which affects the performance and function of the devices.
In silicon oxide films for use as layer insulating films which constitute multilayer wires, water released from the films causes corrosion of aluminum wires, reduction of pressure resistance between the aluminum wires, etc. Therefore, the plasma CVD method is a suitable method for forming a silicon oxide film which contains less water.
The following is a description of a case in which a silicon oxide film is formed by the plasma CVD method. An organic silicon source, e.g., a combination of TEOS (tetraethyl orthosilicate) and oxygen gas (O.sub.2 gas), is introduced into a CVD apparatus of the parallel flat plate type. As a result, a plasma is generated to form a film. In the CVD apparatus of this type, the processing gases are introduced into an airtight processing chamber which is provided with an electrode, doubling as a wafer mount, and an opposite electrode opposed thereto in parallel relation. A radio-frequency (RF) electric field is generated between these electrodes, and the processing gases are converted into a plasma. At the same time, a wafer is heated by means of a heater which is contained in the wafer mount, whereby the processing gases are decomposed. Thus, the filicon oxide film is formed.
The silicon oxide film (hereinafter referred to as P-TEOS film), thus formed by the plasma CVD method using TEOS, contains very little water. However, this film is subject to a drawback that the filling configuration of depressions is awkward.
As the capacity of DRAMs increases, 64M, 256M, and 1 G, wiring gap patterns are reduced to superfine versions of 0.35 .mu.m, 0.25 .mu.m, and 0.13 .mu.m. It is difficult for some CVD apparatuses of the parallel flat plate type to fill the depressions of these superfine patterns without producing voids.
Conventionally, therefore, the depressions are first filled with a P-TEOS film 11 which has good insulating properties, as shown in FIG. 16. In order to secure the surface levelness, moreover, SOG (spin-on-glass) 12 is applied to the resulting structure. In order to seal the SOG 12, furthermore, another P-TEOS film 11 is formed thereon. A TiN film 13 is formed to separate upper and lower wiring structures.
The depressions may be filled to some degree with the P-TEOS film, and thereafter, instead of applying the above method applying the SOG, an alternative method is investigated such that the depressions are filled by forming a silicon oxide film (TEOS-O.sub.3 film) by the hot CVD method using TEOS and ozone. Although the TEOS-O.sub.3 film contains much water, its filling configuration is good enough. The method using this film, therefore, is an effective method.
In this method for forming the plasma CVD film, ions in a plasma serve to remove impurities, thereby restraining the impurities from being caught in the film. In forming a P-TEOS film, for example, carbon (C) and hydroxyl groups (OH) are produced and confined as impurities to the P-TEOS film as TEOS is decomposed to form a film on a wafer. As these impurities get hit by oxygen ions in the plasma, however, they are separated in the form of carbon dioxide or water molecules from the film, and are discharged from the system.
In the conventional CVD apparatus of the parallel flat plate type, however, if radio-frequency power is set within a range such that it exerts no bad influence on the wafer surface, electric discharge cannot occur unless the total gas pressure during plasma generation is as high as about 1 Torr or at some 10.sup.-1 Torr at the least. Therefore, the probability of generated ions in a gas phase to run against one another is so high that the ions are coupled again into gas molecules, and the plasma density is as low as about 1.times.10.sup.10 /cm.sup.3. Accordingly, the ions are few, and the efficiency of ion application to the wafer surface is poor.
As a result, the effect of removal of impurities is small, the film quality is poor, and corrosion of the aluminum wires is accelerated. In consequence, the P-TEOS film is so thin that current leakage and reduction of pressure resistance between the wires are liable to be caused. Thus, the reliability of the devices is lowered.
In order to improve the efficiency of ion application to the wafer surface, it is effective to use an ECR plasma source which can enjoy a high plasma density of about 1.times.10.sup.12 /cm.sup.3. However, an ECR plasma apparatus, which requires use of a large-sized magnet and the like, entails high costs on account of its construction.
In the CVD apparatus of the parallel flat plate type, as described above, the gas pressure must be relatively high. Accordingly, there are many particles in the processing chamber, and SiO.sub.2 and impurities adhere to the inner wall of the processing chamber. In consequence, washing operation must be performed at high frequency, e.g., with every cycle of film formation, thus lowering the throughput.
The SOG, which contains many OH groups, provides a poor film quality, resulting in corrosion or cracking of aluminum and stress migration. If the SOG is applied, then it requires processes for baking, application, and sintering, and besides, repeated execution of P-TEOS film formation, thus entailing an increase in the number of processes of operation. This problem also applies to the case where the TEOS-O.sub.3 film is used.